Plants spike nectar with caffeine and give bees a buzz

The coffee shop may promise productivity and snacks, but only one thing gets you in the door: caffeine. It turns out that honeybees fall for the same trick – and when they do, they probably make less honey.

That might seem like a fair arrangement. “We thought caffeine could make the honeybees efficient pollinators,” says Margaret Couvillon at the University of Sussex, UK.

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But according to her team’s latest research, the caffeine actually leads to behavioural changes that serve the plant’s needs while making the bee colony less productive. “What I think it does is make them exploited pollinators,” she says. “The plants are tricking them into foraging in ways that benefit the plant, not the bee.”

Initially, Couvillon’s team set out to study whether bees given lightly caffeinated sugar water – with concentrations found in nectar – would keep returning to the same location. That required them to keep track of individual bees. “We glued these number tags on their backs, like little football jerseys,” she says.

In search of a buzz

After three hours, those caffeinated feeders ran dry, but the bees’ brief exposure to caffeine had left a lasting mark.

“These poor bees came back for four or five days afterwards, and they were kind of desperate,” Couvillon says. Instead of looking for other sources of nectar, the bees wasted time searching for the caffeine buzz.

But the team soon realised that the caffeine was also warping other foraging behaviours. Honeybees are very sensitive to how sugary their food is, and will do a “waggle dance” once they return home to direct fellow workers back to a particularly sweet source.

Perhaps unsurprisingly, bees coming back from a caffeinated feeder had the jitters – they were more enthusiastic dancers than bees coming back from an equally sugary but decaf beverage.

Couvillon’s team calculates that caffeine quadruples the number of waggle dances a bee will do. And each bee convinced by the dance will come back from the sugar source caffeinated, too, strengthening the effect.

The team then showed in a model that this could lead to less honey production for the colony, as plants could produce less nectar but still attract the bees. All of this sounds suspiciously like cheating, as it could save the plant energy at bees’ expense. “It’s very cheap to maybe throw in a little caffeine into the nectar,” Couvillon says.

No true innocents

“It really raises a lot of interesting questions about the role of caffeine in the food web,” says James Nieh of the University of California in San Diego.

We should remember that in partnerships like these, there are no true innocents, he says. “Although it may seem that the poor bees are being tempted or drugged by these plants, bees have their own tricks that the plants need to defend against,” Nieh says.

For example, bees are known to gnaw their way through the base of a flower to get to nectar, avoiding the pollen they are supposed to carry. That lets bees get nectar from long flowers that their tongues couldn’t normally reach into.

Over time, such relationships co-evolve, keeping cheaters from ever gaining an overwhelming edge. “Nature is not big on honesty unless it’s somehow enforced,” Nieh says. “It’s kind of an arms race.”